This invention relates to an apparatus and a method for measuring the degree of refining of pulp fibers in a pulp suspension in the preparation of furnish for use in paper making, and more particularly to such a device and method for measuring certain properties of the pulp suspension, as modified by the refining process, to allow prediction of the physical properties of the to-be-manufactured paper.
In the making of paper, the fibers are produced by a pulping process which provides what is often referred to as either a stock preparation or a pulp suspension. In order to prepare such pulp suspension to generate a furnish, and in addition to such operations as repulping and blending of pulps of different types and the addition of various chemicals and fillers, it has been found that a mechanical treatment is necessary to make the fibers suitable for forming into sheets of paper. This mechanical treatment is referred to as refining. The refining process is fully described in chapter 4 of the second edition of volume III entitled "Papermaking and Paperboard Making" of a three volume work entitled "Pulp and Paper Manufacture."
One of the common methods heretofore employed to measure the degree of refining to which a pulp suspension has been subjected is the use of the freeness test which is based on the rate at which water would drain from a pulp suspension through a wire mesh on which the fibers are retained in the form of a loose fiber mat. It was found that the longer the pulp suspension was refined, the slower the water drained through the fiber mat. The main value of a freeness test lies in the relationship which the paper makers have found between this test taken on a stock prepared in their equipment and the behavior and characteristics of the sheet-forming process on a paper machine.
One of the disadvantages of the freeness test is that it is strongly influenced by the presence and concentration of fines, less so by the physical condition of the fibers. Consequently, correlation between the freeness test and the physical properties of the paper are greatly dependent on the fines present in the pulp suspension and since the fines do not add materially to the physical properties of the paper, the freeness test is at best imperfect.
Another disadvantage of the freeness test is that it more completely measures one of two important actions of the refining process, i.e., one of two separate actions on the fibers. The first action is external fibrillation which causes an increase in the concentration of fine particulate material, separating this from the fibers by abrasion. External fibrillation has an important effect on the quality of the paper, but is only of secondary value in developing good tensile strength properties. The second action is internal fibrillation to develop internal surface area, causing the fibers' internal bonds to be ruptured so that they become soft and swollen. The degree of internal fibrillation can be correlated with the increase in the specific volume of the fiber and improvement in the tensile strength properties of the paper. The freeness test in common use today is more a measure of external than internal fibrillation, and therefor is not an accurate measure of the more important pulp suspension qualities.